The structure of the DNA helix within eukaryotic cells imposes certain topological problems that the cellular apparatus must solve in order to use its genetic material as a template. The separation of the DNA strands is fundamental to cellular processes such as DNA replication and transcription. Since eukaryotic DNA is organized into chromatin by chromosomal proteins, the ends are constrained and the strands cannot unwind without the aid of enzymes that alter topology. It has long been recognized that the advancement of the transcription or replication complex along the DNA helix would be facilitated by a swivel point which would relieve the torsional strain generated during these processes. Topoisomerases are enzymes that are capable of altering DNA topology in eukaryotic cells. They are critical for important cellular functions and cell proliferation.
There are two classes of topoisomerases in eukaryotic cells, type I and type II. Topoisomerase I is a monomeric enzyme of approximately 100,000 molecular weight. The enzyme binds to DNA and introduces a transient single strand break, unwinds the double helix (or allows it to unwind), and subsequently reseats the break before dissociating from the DNA strand.
Topoisomerase II consists of two identical subunits of molecular weight 170,000. Topoisomerase II transiently breaks both strands of the helix and passes another double-strand segment through the break.
Camptothecin is a water-insoluble, cytotoxic alkaloid produced by Camptotheca accuminata trees indigenous to China and Nothapodytesfoetida trees indigenous to India. Camptothecin and a few close congeners thereof are the only class of compounds known to inhibit topoisomerase I.
Inhibition of topoisomerase II is the major target of important commercial oncolytic agents (e.g., etoposide, doxombicin and mitoxantrone) as well as other oncolytic agents still undergoing development. Camptothecin and its known congeners have no effect on topoisomerase II and none of the known topoisomerase II inhibitors has any significant effect on topoisomerase I.
Camptothecin and most of its analogs have not proven to be attractive for clinical drug development as cytolytic agents because of unacceptable dose limiting toxicity, unpredictable toxicity, poor aqueous solubility, unacceptable shelf life stability, and/or lack of clinical efficacy. ##STR2##
However, water soluble camptothecin analogs having efficacy as topoisomerase I inhibitor antineoplastic agents are known. U.S. Pat. No. 5,004,758, issued to Boehm, et at. on Apr. 2, 1991, the specification of which is incorporated herein by reference, discloses water soluble camptothecin analogs, preferably topotecan (9-dimethylaminomethyl-10-hydroxycamptothecin), preferably (S)-topotecan, of formula: ##STR3## most preferably as the hydrochloride salt. In clinical tests, topotecan has demonstrated efficacy against several solid tumor cancers, particularly ovarian cancer and non-small cell lung carcinoma in humans.
Masuda, et al., J. Clin. Oncology, 1992, 10, 1225-1229 describes CPT-11 (S)-[1,4'-bipiperidine]-1'-carboxylic acid,4,11-diethyl-3,4,12,14-tetrahydro-4-hydroxy-3, 14-dioxo-1H-pyrano[3',4':6,7]indolizino[1,2-b]quinolin-9-yl However, efforts to develop CPT-11 as an antineoplastic agent have been hampered by an adverse toxicity profile.
Wall, et al., J. Med. Chem., 1993, 36, 2689-2700 describes 9-aminocamptothecin ((S)-10-amino-4-ethyl-4-hydroxy-1H-pyrano[3',4':6,7]indolizino[1,2-b]quino line-3, 14(4H,12H)-dione). However, this compound possesses limited water solubility which has posed formulation and bioavailability problems in its development as an antineoplastic agent.
There is a need for new topoisomerase I inhibiting agents which avoid the undesirable features described above. The compounds of the present invention satisfy such need.